SPRINGFIELD, Mo. (AP) _ Antibiotics in chicken feed have long been targeted by critics as a health issue, but a new study by Johns Hopkins University researchers says they also are a money-loser for poultry farmers.

The study in this month's edition of the journal Public Health Reports is billed as the first economic analysis of the cost-benefits of feeding poultry antibiotics to boost growth, a widespread practice since the 1950s.

Groups including the World Health Organization and the American Medical Association have argued that growth-promoting antibiotics, or GPAs, for poultry should be banned. They say the practice can lead to more antibiotic-resistant infections in humans.

Using data from a study of 7 million chickens published previously by Perdue Farms, a major U.S. producer, the Johns Hopkins researchers found that the drugs helped promote growth but cost about a penny per chicken more than the market value the birds gained from antibiotics.

``The net effect of using GPAs was a lost value of $0.0093 per chicken,'' the study concluded. ``Based upon these data, the authors found no basis for the claim that the use of GPAs lowers the cost of production.''

The National Chicken Council, a trade group of major poultry companies, said the study was flawed because it applied average figures for costs that in fact vary across the country.

It said poultry producers use antibiotics primarily to treat disease in animals or to promote animal health, not for growth.

``There has never been a proven case in which human health problems resulted from the use of antibiotics in chickens,'' the council said in a statement.

The study focused on cost benefits and shows there is no financial advantage in using them, said The Keep Antibiotics Working coalition, which opposes the use of the drugs in animal feed and publicized the study.

``But the fact that in today's poultry operations growth-promoting antibiotics don't even reduce costs just strengthens the case for eliminating them. It doesn't make sense _ or cents _ to continue this practice,'' Richard Wood, steering committee chair of the group, said in a statement.

The coalition said Perdue and three other large poultry producers _ Tyson, Gold Kist and Foster Farms _ say they no longer use antibiotics to promote growth.

Those four companies account for more than 38 percent of the broiler chickens produced in the U.S. in 2005, the latest figures available from Watt Poultry USA's annual top broiler company survey. Broiler is the industry term for chickens raised for meat.

The use of antibiotics for growth is still substantial in the poultry industry, said Rebecca Goldburg, senior scientist for Environmental Defense, a member of the coalition.

However, because there are no government limits or requirements for reporting, there are no precise numbers on how much of the industry uses antibiotics for growth.

``The concern is that the more you use antibiotics, the more bacteria begin to tolerate and resist them, so that these drugs become less effective when they're needed to treat critical medical conditions in people,'' Goldburg said.